/**
 *    Copyright 2009-2016 the original author or authors.
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *       http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */
package org.apache.ibatis.builder.xml;

import java.io.IOException;
import java.io.InputStream;
import java.util.Locale;

import org.apache.ibatis.io.Resources;
import org.xml.sax.EntityResolver;
import org.xml.sax.InputSource;
import org.xml.sax.SAXException;

/**Offline entity resolver for the MyBatis DTDs
 * <br/>目的是未联网的情况下也能做DTD验证，实现原理就是将DTD搞到本地，然后用org.xml.sax.EntityResolver，最后调用DocumentBuilder.setEntityResolver来达到脱机验证
 * <br/>EntityResolver
 * <br/>public InputSource resolveEntity (String publicId, String systemId)
 * <br/>应用程序可以使用此接口将系统标识符重定向到本地 URI
 * <br/>但是用DTD是比较过时的做法，新的都改用xsd了
 * <br/>这个类的名字并不准确，因为它被两个类都用到了（XMLConfigBuilder, XMLMapperBuilder）
 *
 * @author Clinton Begin
 */
public class XMLMapperEntityResolver implements EntityResolver {

	// <?xml version="1.0" encoding="UTF-8" ?>
	// <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
	// "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
	// 常量定义
	private static final String IBATIS_CONFIG_SYSTEM = "ibatis-3-config.dtd";
	private static final String IBATIS_MAPPER_SYSTEM = "ibatis-3-mapper.dtd";
	private static final String MYBATIS_CONFIG_SYSTEM = "mybatis-3-config.dtd";
	private static final String MYBATIS_MAPPER_SYSTEM = "mybatis-3-mapper.dtd";

	private static final String MYBATIS_CONFIG_DTD = "org/apache/ibatis/builder/xml/mybatis-3-config.dtd";
	private static final String MYBATIS_MAPPER_DTD = "org/apache/ibatis/builder/xml/mybatis-3-mapper.dtd";

	/*
	 * Converts a public DTD into a local one
	 *
	 * @param publicId The public id that is what comes after "PUBLIC"
	 *
	 * @param systemId The system id that is what comes after the public id.
	 *
	 * @return The InputSource for the DTD
	 *
	 * @throws org.xml.sax.SAXException If anything goes wrong
	 */
	// 核心就是覆盖这个方法，达到转public DTD到本地DTD的目的
	@Override
	public InputSource resolveEntity(String publicId, String systemId) throws SAXException {
		try {
			if (systemId != null) {
				String lowerCaseSystemId = systemId.toLowerCase(Locale.ENGLISH);
				if (lowerCaseSystemId.contains(MYBATIS_CONFIG_SYSTEM)
						|| lowerCaseSystemId.contains(IBATIS_CONFIG_SYSTEM)) {
					return getInputSource(MYBATIS_CONFIG_DTD, publicId, systemId);
				} else if (lowerCaseSystemId.contains(MYBATIS_MAPPER_SYSTEM)
						|| lowerCaseSystemId.contains(IBATIS_MAPPER_SYSTEM)) {
					return getInputSource(MYBATIS_MAPPER_DTD, publicId, systemId);
				}
			}
			return null;
		} catch (Exception e) {
			throw new SAXException(e.toString());
		}
	}

	private InputSource getInputSource(String path, String publicId, String systemId) {
		InputSource source = null;
		if (path != null) {
			try {
				InputStream in = Resources.getResourceAsStream(path);
				source = new InputSource(in);
				source.setPublicId(publicId);
				source.setSystemId(systemId);
			} catch (IOException e) {
				// ignore, null is ok
			}
		}
		return source;
	}

}